The economics of powering microbial metabolism with electrons derived from cathodes is rapidly improving as the cost of renewable sources of electricity declines. Many proof-of-concept studies have demonstrated the potential for microorganisms to convert carbon dioxide or waste streams into organic commodities with electrons derived from cathodes. However, the rates of electron transfer from cathodes into biofilms have been more than an order of magnitude lower than the rates of electron transfer in the reverse direction when biofilms produce electric current on anodes.
As the availability of renewable sources of electricity rises, feeding microorganisms electrons with an electrode is becoming an increasingly attractive possibility for the production of biofuels and other organic commodities, as well as for bioremediation of organic and metal contamination.
In many studies, H2 produced at the cathode functions as an electron shuttle between the cathode and microbes. Supplying H2 to microorganisms in this manner is a relatively old technology. There are well-developed models for the growth and metabolism of diverse anaerobes with H2 as the electron donor.
Less explored is the possibility of developing biofilms that directly accept electrons from cathodes. Direct electron transfer has several potential benefits over the production of H2, such as lower energy inputs, higher recovery of electrons in products, and the retention of the microbial catalyst as a biofilm. However, without a genetically tractable model microbial system that effectively grows on cathodes there has been no knowledge base of first principles to guide the design of cathode microbial catalysts.
Insights into electron transfer within cathode biofilms have recently been obtained through omics and electrochemical characterization of a multi-species oxygen-reducing biofilm enrichment known as the Marinobacter-Chromatiaceae-Labrenzia (MCL) biocathode. However, none of the microorganisms in the MCL biofilm has been recovered in pure culture, which has prevented definitive genetic and biochemical investigations into hypothesized electron transport mechanisms. Genetically tractable isolates are also required for the introduction of synthetic pathways to produce high value products. High recovery of electrons in such products is only possible under anaerobic conditions. Anaerobes that are genetically tractable and are also thought to directly accept electrons from cathodes include Geobacter sulfurreducens, G. metallireducens, Shewanella oneidensis, and Clostridium ljungdahlii. None of these microorganisms grows well on cathodes.